The invention relates generally to signal propagation delay estimation, and more particularly to signal propagation delay estimation using time domain techniques.
Emergency 911 service is rapidly becoming essential in today""s society. One of the compelling reasons for using the existing landline emergency 911 system is the ability to trace the caller""s location. Using databases in the telephone network switches, the caller""s location is determined and made available to the emergency services. In the event the caller is unable to inform the operator of their location, the ability to trace the call is invaluable.
The explosive growth of mobile phones, however, causes complications for emergency 911 services. While mobile users may call the 911 operator just as they would use a landline phone, there is no ability to trace the exact location of the mobile caller. The emergency 911 operator currently can only trace the mobile call to the base station closest to the mobile caller is using.
Mobile systems with the ability to locate mobile callers are known as wireless enhanced 911 or E911 systems. One known approach to determine a mobile caller""s location involves using an improved handset. The improved handset may incorporate a global positioning systems (GPS) receiver to determine the mobile caller""s location and then transmit the location to the emergency 911 operator. Another improved handset may use signals from multiple base stations to determine the mobile caller""s location. These handset improvements, however, involve improved handset circuitry that increases the cost of the handsets. Further, the extra circuitry requires extra battery power. Moreover, deployment of the improvement takes time since it depends on the users upgrading their handsets.
Another approach would not modify the handsets, thereby avoiding the problems stated above. The so-called network approach involves modifying the base stations. One such approach involves angle of arrival techniques using improved antenna arrays at the base station. Another approach uses a rough idea of the mobile""s transmission characteristics and compares these to a large database of the surrounding environment to determine a rough idea of the mobile""s location. In the first approach, however, new antennas must be installed which may be expensive. In the second approach, maintaining the database is difficult since the environment changes readily.
There is, however, a network based approach that involves a minimum amount of hardware upgrade and does not require knowledge of the surrounding environment. The approach is known as Time Difference Of Arrival or TDOA. One method of TDOA involves measuring the Time Of Arrivals (TOA) of the mobile""s signals at multiple base stations. The TOAs are then sent to the Central Office and subtracted to get the measurements of TDOA between base stations. These TDOA parameters are then used to locate the mobile""s position using standard hyperbolic locating techniques.
In known TDOA systems computing the TOA is computationally intensive. To determine a TOA, all base stations must determine the time the same portion of the mobile""s signal is received. Although most of the mobile""s transmission is unknown conversation, the code sequence (shortened burst) used to set up the call is known. Therefore, the base station determines when the call set-up code is received by correlating the received signal with a stored version of the call set-up code. This computation is intensive, and usually requires hardware upgrades in the base station. Furthermore, if the TDOA computation was performed in the Central Office, the links between the base stations usually have to be upgraded for increased bandwidth.
Clearly, there is a need to estimate the Time Of Arrival for mobile signals using a minimum amount of computations and thus minimize the hardware upgrades required for networks implementing enhanced 911.
The invention is directed to a method and apparatus for estimating the time of arrival of a received signal with a reduced amount of computations.
According to one aspect of the present invention, there is provided a time delay estimation method. The method comprises the steps of: (a) estimating a channel response from a received signal, wherein the channel response is estimated from a framed signal according to the formula: G=A+X, where G=the estimated even or odd channel response vector, A+=(AHA)xe2x88x921AH, a pseudoinverse of A (the transmitted data matrix), X=the framed even or odd signal vector; and (b) determining a time delay estimate by correcting the estimated channel response with an ideal channel response.
According to another aspect of the present invention, there is provided an intra-symbol delay estimation method. The method comprises the steps of: (a) estimating a channel response from a received signal, wherein the channel response is estimated from a framed signal according to the formula: G=A+X, where G=the estimated even or odd channel response vector, A+=(AHA)xe2x88x921AH, a pseudoinverse of A (the transmitted data matrix), X=the framed even or odd signal vector; and (b) determining an intra-symbol delay estimate by correlating the estimated channel response with an ideal channel response.
According to another aspect of the present invention, there is provided an intra-symbol delay estimation method. The method comprises the steps of: (a) estimating a channel response from a received signal; and (b) determining an intra-symbol delay estimate by correlating the estimated channel response with an ideal channel response; the step (b) comprising steps of: (i) partitioning the ideal channel response into a plurality of interleaved subsets, with each subset containing half symbol interval spaced samples and having different timing phase, which is indicated by subset index; (ii) determining a correlation result for each subset by correlating each subset with the estimated channel response; and (iii) determining the intra-symbol delay estimate by selecting the highest correlation result.
According to another aspect of the present invention, there is provided a time of arrival estimation method. The method comprises the steps of: (a) matched filtering an input signal to produce a matched filtered signal; (b) buffering the matched filtered signal and adding global positioning system time stamps to produce a buffered signal; (c) compensating for a carrier frequency offset in the buffered signal to produce a received signal; (d) determining the start of a shortened burst signal in the received signal using a framer; (e) from the start of the shortened burst, checking the GPS time and determining an inter-symbol delay; (f) estimating a channel response from the shortened burst; (g) determining an intra-symbol delay estimate by correlating the estimated channel response with an ideal channel response; and (h) determining a time of arrival signal by adding the inter-symbol delay and the intra-symbol delay.
According to another aspect of the present invention, there is provided a system, which comprises: (a) a channel estimator for estimating a channel response from a received signal, the channel estimator being adapted to estimate the channel response from a framed signal according to a formula: G=A+X, where G=the estimated even or odd channel response vector, A+=(AHA)xe2x88x921AH, a pseudoinverse of A (the transmitted data matrix), X=the framed even or odd signal vector; and (b) a correlator for estimating a time delay using the channel response and an ideal channel response.
An advantage of the invention is reduced computational requirements due to using the estimated channel response rather than the received signal in the correlation operation. The reduction in computations reduces the cost and complexity of determining the time delay estimate.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.